Improved description of the magnetocaloric effect in the double exchange model

The double exchange model is used to describe different magnetocaloric materials containing localized magnetic moments and itinerant electrons. The model includes the Hund rule exchange JJ between the localized spins and the conduction electrons. To describe the thermodynamic properties of the model, we follow the same procedure adopted in a previous paper within a mean field approximation. By using the equation of motion method, we apply a higher-order decoupling for the conduction electron Green’s functions. The magnetism of the localized moments is described in terms of an effective Heisenberg model. We obtain a simple description of the magnetization curves and the isothermal entropy change ΔS. The results exhibit an additional low-temperature bump in the ΔS curves at low concentrations nn separated from the usual maximum at the critical temperature. The method can also be addressed to the Kondo lattice compounds, in the case of a negative couping JJ.